FRANKFURT—German unions are accustomed to wielding formidable influence. Union officials sit on supervisory boards at blue-chip companies. They have a track record of wearing down foreign employers.

Not so at Amazon.com Inc. For the 16 years the online retailer has done business in Germany, it has shunned the nation's consensus-driven labor model. It ignores trade unions and largely dictates contract terms at its nine German distribution centers, where it employs about 9,000.